Slurry pumps are often subjected to severe shock loading and shaft whip due to the presence of solids and system upsets. For these reasons soft compression packing is still favored as a means of sealing at the stuffing box.
The preferred method for packing a slurry pump is the “flush” seal shown in Figure 13-5a. Here the lantern ring is positioned in front of the packing rings and a copious supply of clean liquid is injected at a pressure higher than the prevailing slurry pressure in the stuffing box. The clean liquid acts as a barrier and prevents the ingress of abrasive particles that cause packing and sleeve wear. The disadvantage of this system is that large amounts of flushing water are required and the pumped product will be diluted. This system is recommended for severe abrasive services.
Figure 13-5. (A) Typical “flush-type” slurry pump stuffing box. Barrier flush prevents abrasive wear. (B) Typical “weep-type” stuffing box. It uses considerably less gland water but is much more susceptible to abrasive wear.
An alternative method for sealing is shown on Figure 13-5b. Here the lantern ring is positioned between packing rings. This configuration is called a “weep” seal. Again, clean liquid should be injected at a pressure higher than the prevailing slurry pressure near the stuffing box. Product dilution is significantly reduced compared to the “flush” seal design. However, the barrier so created is not very effective, causing abrasive particles to penetrate and cause wear. If the service is only mildly abrasive, then grease can be used in lieu of liquid.
An approximation of flushing requirements for a “flush” type packing arrangement for conventional throat restriction devices where no attempt has been made to curtail the use of flushing water and where the pressure differential is 15 psi is displayed in Figure 13-6. Such a restriction will have an annular radial clearance in the order of .007 times the sleeve diameter. The length of the throat bush will be about the same as the width of one turn of packing.
Figure 13-6. Approximation of flushing water requirements for “flush-type” slurry pump stuffing boxes where flush pressure is 15 psi above slurry pressure at the box. “Weep-type” stuffing boxes use about 5% of the water that “flush-types” use.
It is impossible to predict the exact amount of flushing water required when the packing is “weep” type, since this is dependent on shaft deflection and gland maintenance. However, under normal operating conditions, weepage would be in the order of 5% of the values stated in Figure 13-6 for “flush” packing arrangement.
In most cases, seals and flush requirements are provided in ignorance of the real pressure prevailing at the stuffing box, which results in excessive use of gland water and increased maintenance.
Built into some slurry pump designs are methods to reduce pumped pressure at the stuffing box by hydrodynamic means. (For example, see Figures 13-7 and 13-8 for diagrams of pump out vanes on impellers and expellers.) The side-suction-pump configuration is subjected only to suction pressure and has an advantage over end-suction pumps, one not fully recognized by users. By proper application of impeller pump out vanes and expellers, the pressure at the box can be reduced to almost zero. This is called a dry box arrangement. In these cases, weep-type seal is satisfactory, with either water or grease being injected into the cavity formed by the lantern ring.
Figure 13-7. End-suction pump fitted with expeller.
(courtesy Goulds Pumps, Inc.)
Figure 13-8. Side-suction pump fitted with expeller.
An abrasive slurry pump is used as one of the main components of the abrasive slurry circulation system to circulate abrasive slurry solution. The abrasive slurry solution consists of abrasive particles mixed with water, which also functions as cooling system for the tool and workpiece. Varying the control valve attached at the mill-module controls the flow rate of abrasive slurry. The power supply on abrasive slurry pump is controlled from main control unit. The performance of machining operations greatly depends on the slurry concentration, slurry flow rate and cooling system of the ultrasonic machine. Higher abrasive slurry concentration may increase higher material removal rate but lower level of concentration is preferable for better functioning of slurry pump.
The most commonly used abrasive is boron carbide (B4C), which is one of the hardest materials that can be used as abrasives for USM operations. Abrasive particle is selected for USM based on hardness, size of it and also the type of workpiece materials. The other types of abrasives are aluminum oxide (Al2O3), silicon carbide (SiC), boron silicarbide and diamond. Water is used as common liquid media to make abrasive slurry. The other liquids such as oils, benzene and glycerol can also be used.
A re-circulating pump as shown in Fig. 3.1.7 forces abrasive slurry to the work material. Usually, abrasive particles are suspended in a liquid medium, such as water, between the tool face and the work piece. The average grain diameters corresponding to abrasive grit size used for experimentation are listed in Table 3.1.1. Grit numbers above 400 onwards result better surface finish but lesser value of material removal rate. Slurry concentration based on selected levels has been taken as 30%, 35%, 40%, 45%, and 50% by volume.
The crushed food waste is delivered to the conditioning tank through a thick slurry pump. In order to achieve the design processing capacity and control the flow of materials in the fermenter and the operation of the equipment, it is necessary to accurately measure the amount of waste entering the conditioning tank. Therefore a continuously meterable device is placed on the pipe to which the slurry pump is connected.
4.6.1.2.2 Material property adjustment
In order to achieve the best microbial degradation conditions, the food waste must be adjusted in the adjustment tank before fermentation into the fermenter. The material is diluted with the reused process water of the dehydration unit and diluted to a solid content of about 10%. The required dilution water is calculated by the central control system and then automatically delivered by the volumetric metering pump. The main purpose of returning water is to maintain the vitality of the nutrients and microorganisms in the fermenter.
The boiler steam is used to heat the material. The amount of steam required is calculated by the central control system based on parameters such as ambient temperature, amount of waste, and amount of diluent. The maximum steam production is designed to be 150 kg/h. The advantage of this heating system is its high adaptability, which maintains the temperature of the material even when the outside temperature is low.
The material must be mixed evenly before entering the fermenter to ensure that the biological reaction process proceeds smoothly. At the same time, a certain amount of the fermented feed liquid is added to the mixture to increase the uniformity of the material and to make the mixture easy to transport. Mixing is done in the conditioning tank. The stirred mixture has the same uniformity as the viscous sludge. It is suitable for microbial degradation and can be pumped directly into the fermenter. The hopper above the agitator provides a uniform flow to the agitator to buffer flow fluctuations during material reception.
4.6.1.2.3 Fermenter feed
The fermenter is fed during the set working hours. When the feed is stopped, the microbial activity in the fermenter does not stop immediately but lasts for several days. A homogeneous mixture in the conditioning tank is injected into the fermenter by a particularly robust piston pump. The material-conveying pipe is made of carbon steel alloy and has thermal insulation and corrosion resistance.
The pH of the SPFRD hydrogen production tank and CSTR methane fermentation tank should be tested online, and can be adjusted at a setting range in time.
The discharge and liquid treatment must also be carried out during working hours, that is, at the same time as the fermenter feed, so that the fermentation can maintain a stable level in the fermenter. The correctness of the inlet and outlet operations can directly affect the stability and safety of the plant operation.
There are a number of slurry erosion test machines discussed in the literature.9,10 Many of these pump slurry at high velocity at the surface of a coated test plaque. A simpler test is described in ASTM G75-07 Standard Test Method for Determination of Slurry Abrasivity (Miller Number) and Slurry Abrasion Response of Materials (SAR Number). The relative effect of slurry abrasivity determined by measuring the mass loss of a block plastic elastomer after it has been driven in a reciprocating motion in a trough containing the slurry. A direct load is applied to the test plaque. The interior of the trough has a flat-bottomed or truncated “V” shape trough that forces the slurry particles to the reciprocating path taken by test specimen. The slurry may be of any material of interest such as sand in water or other liquid. The Miller number machine lifts slightly the test block and delays momentarily at the end of each stroke to allow time for fresh slurry material to flow back into the wear path. The test consists of measuring the mass loss of a part per unit time.
Fabrication and Processing Errors Can Prove Costly
There is an interesting story behind a long series of randomly occurring thrust-bearing failures in one particular type of slurry pump in service at a South American bauxite mine. Apparently the thrust bearings would sometimes fail after a few days or, at other times, after a few weeks of operation. Before the mechanics produced a crossectional view similar to the simplified version depicted in Figure 9-4, the visiting troubleshooter had been told that it was often necessary to rebush and line-bore the bearing housing. The relevance, accuracy, or importance of this verbal description becomes evident only when the drawing is examined in detail.
Figure 9-4. Cross-sectional view of slurry pump with failure-prone thrust bearing configuration.
With the impeller inverted so as to reduce the differential pressure across the shaft packing area, it is immediately shown that the primary thrust is from right to left. The two angular contact bearings on the extreme left are correctly oriented to take up the predominant load. However, their outer rings are completely unsupported because the fabricator had somehow decided to overbore the housing in the vicinity of these two bearings. Consequently, the entire radial load acting on the coupling end of the pump had to be absorbed by the remaining third angular contact thrust bearing. This bearing was thus overloaded to the point of rapid failure and was, of course, prone to rotate in the housing. Using a double row spherical roller bearing at the hydraulic end of the pump would normally make for a sturdy, well-designed pump. In this case, however, the spherical rotation or compliance feature tended to further increase the radial load transferred to the one remaining outboard bearing. The basic agent of the component failure mechanism was, of course, force.
An equally serious burden was imposed on this pump by the well-intentioned person who, in an effort to link the spare parts requirements of the North and South American plants of this major aluminum producer, added to the drawing the parts list partially reproduced in the lower left-hand corner of Figure 9-4. Having left off the appropriate alphanumeric coding behind the bearing identification number 7312, this plant and its sister facilities would receive thrust bearings in other than matched sets. A quick look at the bearing manufacturer's dimension tables (see insert, Figure 9-4) shows simple type 7312 bearings to have a width that may differ from the next bearing by as much as 0.010 inches. Mounting two such bearings in tandem may cause one to carry zero to 100% of the load, while the other one would simultaneously carry 100 to zero % of the load. On the other hand, matched sets intended for tandem mounting would be precision-ground for equal load sharing (50/50%) and would be furnished with code letter suffixes to indicate this design intent.
Did we go through our seven cause categories to identify the above root causes? Frankly, no. When both the fabrication sketch and the procurement documentation— “information processing”—show two very obvious errors, it is reasonable that rectification of these deviations should be a prerequisite to further fine-tuning. Which is just another way of saying that if it looks like a duck, walks like a duck, and quacks like a duck, we ought to call it a duck and dispense with further research into the ancestry of the bird.
Slurry shield construction method is the method of tunnel construction by using slurry shield. The principle is the slurry feed pump(s) pumps slurry (a mixture of water and bentonite) from the slurry preparation tank set on the ground and sends slurry to slurry chamber through a slurry feed line. In the slurry chamber, the pressurized slurry penetrates into the ground a few centimeters deep, and bentonite is pushed into the gaps of soil grains, forming a layer of “cake” so the soil layer of the excavation face becomes more stable and impermeable. By rotation of the cutting wheel of the slurry shield, the “cake” formed in the excavation face is cut down and mixed with the bentonite slurry in the slurry chamber, and then through the slurry pump and the booster pump(s) in the tunnel, is transported to the slurry separation plant on the ground, the slurry separation plant separates the excavated muck from the bentonite slurry, and the separated bentonite slurry can be recycled after quality adjustment, where it is pumped into the slurry chamber of the shield machine by slurry feed pump(s); see Fig. 3-3.
Issues for slurry feed and dry feed systems tend to be different. One main issue related to the slurry feed system is settlement of particulate matter in both storage tanks and suction pipes (especially upstream of the slurry pump during downtimes). This can be avoided by ensuring permanent motion of the slurry. Most slurry fed IGCCs employ slurry storage tanks, which can bridge unplanned downtime of the rod mills. Moreover, most plants (except Tampa) have two slurry pumps of 50–100% capacity each.
A similar decision is necessary for dry fed systems. The number and capacity of mills have a decisive influence on the availability of the fuel preparation unit. The 2×60% roller mills from Puertollano have been identified as being insufficiently robust (Peña, 2005) and to be important contributors to outages. The 3×50% mills used in Buggenum ensure increased availability of the fuel preparation unit. Dry fed IGCCs are of course concerned with common issues related to transport and storage of ground coal, e.g. bridging in sluicing devices or clogging of conveyors as experienced at Puertollano. Moreover, it is not trivial to maintain a stable fluidization and adequate pressure control in dense phase transport systems and to establish a vital and reasonable coal dust explosion prevention system.
An issue common to both feed systems is the proper blending of raw materials in order to guarantee adequate and predictable characteristics of the gasifier feedstock. This is of decisive importance, since adjustment of gasification conditions, slag removal and raw gas cooling parameters is based on predicted feedstock properties.
9.3.8 Fossil-fired flue gas and CO2 scrubber pumps
Flue gas desulfurization scrubbers usually involve causing the flue gas to circulate through thin lime slurry in a contactor. The flow rates are large and the limestone slurry is abrasive. Depending upon customer preference, normal centrifugal or rubber-lined slurry pumps are commonly used.
CO2 scrubbers are similar but may use one of a variety of different solvents. Ammonia and various recipes of amine are used. As above, pressures are relatively low and flow rates are high. CO2 and water form carbonic acid so pump materials are typically 300 series SS as a minimum. The rich solvent returns from the contactor to a stripper where it is heated to drive off the gases and the solvent is recycled.
Compressors are then used to boost the CO2 to supercritical pipeline pressures (>90 bar or 1300 psi). If the local seawater temperature is low enough, it may be more efficient to cool the CO2 to a dense phase and pump it to the higher pressure needed for long pipelines and for injection.
CO2 has a very low viscosity even at supercritical pressures so pipeline friction losses are relatively minimal. CO2 has a surface tension less than 10% of that of propane, so careful attention is paid to welding and casting integrity.
The CO2 can be injected into oilfields to enhance oil production. It can also be pumped into salt domes or hard rock for sequestration.
Understanding compressible supercritical fluid performance in pumps is important. Mechanical seal technology is also important.